The present invention relates to a magnetic recording system, and more particularly, is directed to a magnetic recording system having a recording density higher than 2 gigabits per one square inches and a thin film magnetic recording medium having a low media noise but having a sufficient stability against a thermal fluctuation in order to become able to realize such a magnetic recording system.
At present, there is an increasing demand that a magnetic recording system becomes larger in storage capacity. As a magnetic head, there has hitherto been used an inductive head which makes effective use of a phenomenon in which a voltage is changed as a magnetic flux is changed with a time. According to the above-mentioned inductive head, a single magnetic head is able to both write and read information in and from a magnetic recording medium.
Recently, there is rapidly spread a combination type magnetic head comprising a record head and a write head and in which a magnetoresistive head of a higher efficiency is used as the read head. The magnetoresistive head makes effective use of a phenomenon in which an electric resistance of a head element is changed in accordance with a change of a leakage flux from a magnetic recording medium. On the other hand, there has hitherto been developed a head of a much higher efficiency which makes effective use of a considerably large magnetoresistive change (i.e. giant magnetoresistive effect or spin valve effect) generated in a magnetic layer of the type such that a plurality of magnetic layers are laminated with each other through a non-magnetic layer. This giant magnetoresistive effect is such one that relative directions of magnetization of a plurality of magnetic layers laminated with each other through the non-magnetic layer are changed by a leakage field from Et magnetic recording medium, thereby resulting in an electric resistance being changed.
To realize a high recording density, it becomes necessary to further reduce a media noise. A study of measured results obtained by a computer simulation and measured results obtained by experiments reveals that, in order to reduce a media noise, it is effective to reduce an exchange interaction between magnetic crystal grains or to reduce a grain size of a magnetic crystal grain (J. Appl. Phys., Vol. 63(8), 3248 (1988), J. Appl. Phys., Vol. 79(8), 5339 (1966)). Specifically, as a method of reducing an exchange interaction, there are mainly enumerated a method of increasing a Cr content in a magnetic layer, a method of increasing a spatial separation of magnetic crystal grains and the like. When the Cr content of the magnetic layer is increased, a larger amount of Cr may be segregated into the crystal grains, thereby decreasing the exchange interaction between the magnetic crystal grains. However, at the same time, the Cr content in the magnetic crystal grains also is increased, thereby resulting in a saturated magnetic flux density being lowered. Therefore, the film thickness of the magnetic layer should be increased in order to maintain a value of Br.times.t which is a product of a residual magnetic flux density Br and a film thickness t of a magnetic layer. However, since the crystal grain increases its size as the film thickness of the magnetic layer increases thereby to cause a media noise to increase, this method has a limit. Moreover, in order to increase the separation of the magnetic crystal grains, it is necessary to reduce a size of crystal grains of each underlayer having an acicular structure. To this end, a film thickness of an underlayer has to be increased. Also in this case, it is unavoidable that a size of a magnetic crystal grain formed on the under-layer is increased. Thus, this method also has a limit.
Further, Japanese laid-open patent application No. 7-311929 describes a method in which oxide such as SiO.sub.2 is added to a magnetic layer, the resultant product is segregated into the crystal grains thereby to reduce an exchange interaction between crystal grains and in which, at the same time, a grain size of a crystal grain ay be decreased by suppressing a crystal growth. However, because the addition of the insulating material causes a resistance value of a target to increase considerably, it is unavoidable that a film is deposited by an RF (radio frequency) sputtering. As compared with a DC (direct current) sputtering, the RF sputtering is inferior to the DC sputtering from a standpoint of a manufacturing cost and a stability or the like, and hence is not suitable for a mass-production. Moreover, since a crystallographic orientation of a magnetic recording medium manufactured by the RF sputtering is difficult to control, there arises such a problem that it is very difficult to obtain a high coercivity and a coercivity squareness.
Furthermore, since the magnetic crystal grain of which the grain size is reduced is much more strongly affected by the influence of the thermal fluctuation, a stability of: a recorded magnetization is lowered considerably. As a consequence, a ratio in which an inversion of magnetization occurs increases with a time so that it becomes unable to maintain a sufficiently high reliability when data is saved during a long period of time. Further, since the crystal grain the grain size of which is reduced is much more strongly affected by a magnetostatic interaction from the adjacent crystal grain, there is unavoidably caused the increase of a media noise. Accordingly, it is desirable that the crystal grain should keep a certain large size.
On the other hand, heretofore, there have been reported a Large number of experiments in which a magnetic layer is formed as a bilayer or a multilayer. Japanese laid-open patent application No. 8-147660, for example, describes a magnetic recording medium having a high coercivity and a low media noise. This magnetic recording medium comprises two magnetic layers in which a first magnetic layer is formed of a CoCrTa alloy layer and a second magnetic layer is formed of a CoCrPtTa alloy layer. However, according to this previously-proposed magnetic recording medium, since an influence exerted by excessively small crystal grains existing at the initial crystal growth portion of the magnetic layer cannot be eliminated, there cannot be expected an effect for suppressing a decay of a read signal due to a thermal fluctuation. The decay of the read signal due to the thermal fluctuation becomes a serious problem particularly when the magnetic layer is reduced in thickness. Also, Japanese laid-open patent application No. 8-77544, for example, describes a magnetic recording medium having a high coercivity in which a magnetic layer has a bilayer laminated structure comprising a soft magnetic layer and a hard magnetic layer. However, if the magnetic layer contains the soft magnetic layer, there is then the large possibility that a media noise will increased due to a strong exchange interaction. Further, such a magnetic recording medium should not be preferable because it is easily affected by an external magnetic field. Furthermore, Japanese laid-open patent application No. 6-243454, Japanese laid-open patent application No. 6-342511 and Japanese laid-open patent application No. 6-349047, for example, describe magnetic recording media in which a magnetic layer is divided by a non-magnetic intermediate layer such as a Cr non-magnetic layer. Since the non-magnetic intermediate layer weakens the exchange interaction between the divided magnetic layers, the size of the magnetization inversion is reduced, and as a result, a media noise can be reduced. However, as is described in IEEE TRANSACTIONS ON MAGNETICS, Vol. 30, pp. 4230-4232 (published in 1994), since a magnetostatic interaction acts on the divided magnetic layers, if the size of the magnetization inversion is reduced to such an extent that it is affected by a thermal fluctuation, there is then the possibility that a magnetization will be canceled by this negative interaction out.
As described above, in the magnetic recording media using the multilayer magnetic layer according to the examples of the related art, although magnetic properties can be improved to some extent but such improvements are not sufficient. Besides, it cannot be expected to suppress the influence of the thermal fluctuation which becomes remarkable particularly when the magnetic layer is reduced in thickness. Therefore, the above-mentioned magnetic recording media according to the related art are not sufficient for realizing a high recording density which is greater than 2 gigabits per square inches.
As described above, in order to realize a high recording density, it becomes necessary to provide a magnetic recording medium which not only has a low media noise but also has a sufficiently high stability against a thermal fluctuation.